/*
 *    gcc -o a a.c -lwiringPi
 */

#include <stdio.h>
#include <wiringPi.h>

int main (void)
{

  if (wiringPiSetup () == -1)
    return 1 ;

  pinMode (28, OUTPUT) ; 

  for (;;)
  {
    digitalWrite (28, 1) ;       // On
    delay (500) ;               // mS
    digitalWrite (28, 0) ;       // Off
    delay (500) ;
  }
  return 0 ;
}